Many modern refineries devote extraordinary amounts of energy and operating expense to convert most of a whole crude oil feed into high octane gasoline. The crude is fractionated into a virgin naphtha fraction which is usually reformed, and gas oil and/or vacuum gas oil fraction which are catalytically cracked in a fluidized catalytic cracking unit (FCC) unit.
A solid cracking catalyst in a finely divided form, with an average particle size of about 60-75 microns, is used. When well mixed with gas, the catalyst acts like a fluid (hence the designation FCC) and may be circulated in a closed flow loop between a cracking zone and a separate regeneration zone.
The Kellogg Ultra Orthoflow converter, Model F, shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal, is an example of a modern, efficient FCC unit. This design (and many other FCC designs not shown) converts a heavy feed into a spectrum of valuable cracked products in a riser reaction in 4-10 seconds of catalyst residence time.
In the cracking zone, hot catalyst contacts the feed to heat the feed, effect the desired cracking reactions and deposit coke on the catalyst. The catalyst is then separated from cracked products which are removed from the cracking reactor for further processing. The coked catalyst is stripped and then regenerated.
A further description of the catalytic cracking process may be found in the monograph, "Fluid Catalytic Cracking with Zeolite Catalysts", Venuto and Habib, Marcel Dekker, New York, 1978, incorporated by reference.
The FCC process is an efficient converter of heavy feed to lighter products, and has some favorable peculiarities. The FCC unit rejects the worst components of the feed as coke and regenerates the catalyst by burning this coke to supply the heat needed for the endothermic cracking reaction. On a volume basis it makes more product than feed. This "swell"--the expanded volume of liquid products after cracking a heavy feed--is one reason the process is so profitable.
Most refiners try to optimize the profitability of their FCC units, either by to maximizing swell or minimizing the yield of low value, heavy fuels. The preferred approach depends on the season. In winter there is considerable demand for heavy fuel and less for gasoline, hence it is usually most profitable to produce more heavy fuel and bottoms fractions off the FCC. In summer, gasoline demand is high and heavy liquid fuels are less valuable. Therefore, refiners usually try to minimize production of heavy fuel fractions such as light and heavy cycle oil and bottom fractions such as slurry oil. These materials, which can simply be looked on as the 650.degree. F. and heavier liquid products, are the least valuable products of an FCC unit. Unfortunately they tend to be produced in significant amounts, especially when poor quality feeds containing large amounts of residual material are fed to the FCC unit.
Refiners have tried to improve yields in catalytic cracking by changing catalyst and reaction conditions. Essentially all refiners currently use zeolite cracking catalyst. In the 70's, use of catalyst with perhaps 10 wt % Y zeolite was common, but today many units use makeup catalyst with 30 to 40 wt % Y zeolite.
Partially in concert with these higher activity catalyst, FCC units have evolved toward ever shorter reaction times. From dense bed cracking, to hybrid units with both dense bed and riser cracking, to modern units with riser cracking alone. Effective reaction time has also been reduced by quick separation of cracked products from spent catalyst exiting the riser. The trend to shorter contact times continues.
Reduction in contact time beyond the point balanced by higher catalyst activity has led to increases in regenerated catalyst temperature. This is also due to the conviction that extremely hot catalyst can "shatter" the asphaltene molecules found in ever larger quantities in today's FCC feeds. Recently --two- stage--regenerators have been developed which achieve regenerated catalyst temperatures of 1400.degree.-1500.degree. F.
The patent literature is replete with references to short contact time cracking, but almost all commercial units today operate with riser reactors, 4 to 10 seconds of catalyst residence time, riser top temperatures of about 950.degree. to 1025.degree. F., and 4:1 to 8:1 cat:oil weight ratios.
In seeking to develop a viable short contact time cracking process, I reviewed internal studies which had investigated cracking at higher temperatures and/or shorter contact times. Much of the work was inconclusive either because contradictory results were obtained in different studies, or because a tradeoff was identified which made it impossible to generalize on the benefits of the new operating conditions. For example, one study showed gasoline selectivity reached an optimum at 3 seconds contact time, but octane was lower by 1 to 3 numbers depending on catalyst. Other work in the 3-7 second contact time range showed that higher temperature reduced both coke and gasoline selectivities, while a follow-up study showed gasoline selectivity increased monotonically at short contact time within the range of 3 to 7 seconds.
From this review I concluded that current cracking conditions, although used for decades in more than 100 FCC units, were not the best. I did additional work in a laboratory FCC riser pilot unit, and discovered that the best way to minimize bottoms yields in an FCC unit was not short contact time at all. I learned that cooler rather than hotter catalyst gave better conversion of heavy feeds, and that longer rather than shorter catalyst residence times were necessary to balance the reactions occurring in the riser. However, the most important factor was to use extraordinary amounts of catalyst--far more than typically used in a commercial FCC unit--at a temperature below that which could be produced in any commercial FCC regenerator. This improbable mix of conditions--more catalyst than had ever been used before, a temperature lower than any modern FCC regenerator can operate at, and preferably a feed inlet temperature exceeding that reachable by conventional FCC feed preheaters--minimized yields of low value heavy products. My process did not require, and in fact would not work with, very short contact times such as less than 1 or 2 seconds catalyst residence time.